The invention relates to a process for the preparation of 4,4'-dihalobiphenyls by selective halogenation o biphenyls in the presence of zeolites as catalysts having pore sizes of at least 5 .ANG..
4,4'-Dichloro- and -dibromobiphenyl are of great interest, for example as intermediates for the preparation of highly heat-resistant plastics, such as modified polyphenylene sulphide (cf. JP Patent Application No. 61/231,030 or U.S. Pat. No. 3,396,110 [C.A. 69 P 60 564 w]).
The conventional chlorination of biphenyl in the presence of Lewis acids leads to an unselective random substitution, in which the 4,4'derivative is not the preferred one. Thus, chlorination at 100.degree. C. in the presence of 2.5% of FeCl.sub.3 produces a selectivity of only 8% of the 4,4'-dichloro isomer; the percentage of trichlorobiphenyl at 15% is remarkably high, the polychlorinated biphenyls belonging to the classes of the highly toxic substances as is known. According to the data of U.S. Pat. Nos. 1,946,040, 3,226,447 and GB No. 1,153,746, the addition of a sulphur compound in the chlorination of benzenes is intended to increase the selectivity in favour of the para-substitution. However, as shown by Comparative Example III below, the selectivity in favour of the 4,4'-dichlorobiphenyl in the chlorination of biphenyl is increased only insignificantly by the addition of 2.5% by weight of thiophene in addition to 2.5% by weight of FeCl3, while polychlorinated biphenyls at more than 9% by weight of the reaction product still constitute a significant percentage.
In practical application, the use of Lewis acids furthermore leads to increased problems with corrosion, as is known, and also to a more complicated workup and disposal.